The objective of this proposed project is to morphologically analyze the sinoatrial node of the mouse heart. This will be accomplished by the study of serial sections of the entire node for both light and electron microscopy with subsequent three-dimensional reconstruction. From electron micrographic montages of the transverse sections of the node and from alternate light microscopic sections, the shape and sizes of the nodal cells, their interrelationships and their pattern of innervation will be determined.